1. Field of the Invention
The present invention relates to a structure for supporting, between a speed-change gear unit and a casing of an automatic transmission for a vehicle, a one-way clutch for preventing reverse rotation of a speed-change gear.
2. Description of the Related Art
An automatic transmission for a vehicle is designed such that through selective supply of pressurized oil to frictional engagement elements such as a clutch and a brake, an arbitrary rotational element within a speed-change gear unit is connected to either an input shaft of the transmission or the casing of the transmission to thereby automatically change the gear ratio in accordance with the traveling state of the vehicle.
Specifically, the crankshaft of an engine is connected to the speed-change gear unit via a torque converter. Torque transmitted from the crankshaft to the torque converter is further transmitted to an input shaft of the speed-change gear unit, which is disposed on the rear side of the torque converter and which constitutes a four-speed transmission having a single reverse gear. The speed-change gear unit is composed of a plurality of sets of clutches and brakes and a single set comprising a one-way clutch and a planetary gear mechanism.
When a driver operates a shift lever disposed in the proximity of a driver's seat of an unillustrated vehicle, the respective frictional engagement elements are brought into an engaged state in accordance with the selected position of the shift lever and the traveling state of the vehicle, and one of the gears is automatically selected by means of hydraulic control unit in response to an instruction from an electronic control unit--which controls the operating state of the engine.
FIG. 5 shows a conventional support structure for a one-way clutch used in an automatic transmission as described above. As shown in FIG. 5, a one-way clutch 101 constituting a speed-change gear unit is disposed between a ring-shaped outer race 104 fixed to a carrier 103 of a planetary gear mechanism 102 and a ring-shaped inner race 106 fixed to a transmission casing 105. The one-way clutch 101 is composed of a plurality of sprags 107 and an end bearing 108 for supporting the sprags 107. Thus, the one-way clutch 101 allows the carrier 103 to rotate in only one direction.
In FIG. 5, numeral 109 denotes an input shaft of the speed-change gear unit; numeral 110 denotes a brake; numerals 111, 112, 113, and 114 respectively denote a ring gear, a short pinion gear, a front sun gear, and a rear sun gear, which constitute the planetary gear mechanism 102; numeral 115 denotes an output shaft of the transmission; and numeral 116 denotes a kick-down drum.
As described above, in the one-way clutch 101, the plurality of sprags 107 are disposed circumferentially between the outer race 104 and the inner race 106. One end of each sprag 107 is in contact with the inner circumferential surface of the outer race 104, while the other end of each sprag 107 is in contact with the outer circumferential surface of the inner race 106. When the rotational direction of the outer race 104 relative to the inner race 106 changes, the inclination angle of each sprag 107 changes in order to allow or prohibit rotation of the outer race 104. In the one-way clutch 101, it is important to maintain a constant distance between the inner circumferential surface of the outer race 104 and the outer circumferential surface of the inner race 106 at all circumferential positions, and therefore the end bearing 108 for supporting the sprags 107 has conventionally been disposed between the outer race 104 and the inner race 106.
In such a conventional structure for supporting the one-way clutch 101, the outer race 104 for supporting the one-way clutch 10 is fixed to the carrier 103 of the planetary gear mechanism 102 connected to the input shaft 109, and the inner race 106 is fixed to the transmission casing 105. The position of the axis of the outer race 104 depends on the assembly accuracy of the planetary gear mechanism 102, while the position of the axis of the inner race 106 depends on the manufacturing accuracy of the transmission casing 105. Therefore, in a case of insufficient assembly accuracy of the planetary gear mechanism 102 or insufficient manufacturing accuracy of the transmission casing 105, there exists an eccentric state in which the axis of the outer race 104 and the axis of the inner race 106 deviate from each other, with the result that the distance between the inner circumferential surface of the outer race 104 and the outer circumferential surface of the inner race 106 varies along the circumferential direction. In such a case, there exists a risk that the sprags 107 do not operate properly in accordance with the rotational direction of the outer race 104.
Although the end bearing 108 is disposed between the inner circumferential surface of the outer race 104 and the outer circumferential surface of the inner race 106 in order to maintain constant the distance therebetween at all circumferential positions, due to a limited installation space therefor the end bearing 108 cannot be increased in size to have an increased rigidity. Therefore, in the above-described case where the outer and inner races 104 and 106 fall in an eccentric state due to insufficient assembly accuracy of the planetary gear mechanism 102 or insufficient manufacturing accuracy of the transmission casing 105, the end bearing 108 wears through sliding contact with both the inner circumferential surface of the outer race 104 and the outer circumferential surface of the inner race 106, resulting in a deterioration in the bearing function. In such a case, there arises a problem that the sprags 107 do not operate properly and become unable to allow or prohibit rotation of the outer race 104 relative to the inner race 106.
A technique that solves the above-described problem is disclosed in, for example, Japanese Utility Model Application Laid-Open (kokai) No. 60-23324. In the "starting clutch for a motorcycle" disclosed in the publication, a one-way clutch composed of a ratchet and rollers is interposed between a crankshaft and a primary drive gear rotatably supported on the crankshaft; a driven gear and a one-way clutch outer race are joined integrally; and female teeth formed on the one-way clutch outer race are engaged with a smaller-diameter portion of teeth at one end of the primary drive gear, wherein at the small diameter portion the tooth height is approximately half that of the remaining portion. Accordingly, the one-way clutch outer race can have some degree of freedom, so as to absorb eccentricity of the one-way clutch outer race caused by a dimensional error in the ratchet portion, thereby enabling proper operation of the one-way clutch.
However, the "starting clutch for a motorcycle" disclosed in Japanese Utility Model Application Laid-Open No. 60-23324 has the following drawbacks. That is, in the above-described one-way clutch, the primary drive gear is supported on the crankshaft; the one-way clutch outer race can move in the radial direction within a distance corresponding to the clearance between the teeth of the smaller diameter portion and the female teeth; and a disc spring is interposed between the primary drive gear and the one-way clutch outer race. Therefore, when the one-way clutch outer race is in an eccentric state, the one-way clutch outer race moves radially relative to the primary drive gear in order to establish alignment therebetween. However, since the driven gear and other components are integrally joined to the one-way clutch outer race, the overall mass of the one-way clutch outer race and other components is large. Therefore, it is difficult for the one-way clutch outer race to move in the radial direction while rotating, resulting in a potential failure to perform alignment reliably.
Further, since the primary drive gear and the one-way clutch outer race are completely separated from each other via a clearance and only the disc spring is interposed therebetween, rotation of the one-way clutch outer race becomes unstable. Moreover, since the alignment of the one-way clutch outer race is performed by the ratchet and the rollers, proper alignment cannot be achieved.